Ukraine's central bank says an investigation into the country's largest lender, PrivatBank, shows that it had been "subjected to a large-scale and coordinated fraud" over at least a decade.

The fraud resulted in the bank, which was taken into state control in 2016, suffering a loss of at least $5.5 billion, the central bank said in a statement on January 16.

One of the bank's former main shareholders, Ihor Kolomoyskyy, dismissed the results of the probe as "nonsense."

The central bank commissioned the independent audit conducted by the global investigation firm Kroll and other international companies.

Central bank deputy governor Kateryna Rozhkova said that the findings have been passed to the Prosecutor-General's Office for use in a possible criminal investigation into the deals made before PrivatBank was nationalized.

PrivatBank’s nationalization occurred in December 2016 with the backing of the International Monetary Fund (IMF), after risky lending practices left it with a capital shortfall of more than $5.5 billion.

At the time, the central bank estimated that 97 percent of its corporate loans had gone to companies linked to Kolomoyskyy and the other main shareholder, Hennadiy Boholyubov.

Boholyubov has said that authorities in Kyiv declared PrivatBank insolvent on grounds that were fabricated and unfounded, creating an "artificial hole" in the balance sheet.

Ukrainian oligarch Ihor Kolomoyskyy (file photo)

Kolomoyskyy, a former regional governor, has turned to the courts to challenge the nationalization, and tried to prevent Ukrainian authorities from cooperating with external companies to investigate the reasons for PrivatBank’s insolvency.

Kolomoyskyy, one of Ukraine's richest men, served briefly as head of Dnipropetrovsk Oblast before President Petro Poroshenko dismissed him in 2015, accusing him of setting up a private militia and trying to take over a state-affiliated oil company.

Kolomoyskyy had been credited with preventing the spread of separatist sentiment in Dnipropetrovsk Oblast following Russia's seizure of Crimea in March 2014 and backing of armed separatists further east in the Donbas.

The region Kolomoyskyy governed, which is now called Dnipro Oblast, borders the Donesk Oblast -- one of two regions held in part by Russia-backed separatists whose conflict with Kyiv's forces has killed more than 10,300 people since April 2014.